Inductive power transmission has many important applications spanning many industries and markets. Although the disclosure contained here contemplates the use of this invention to applications requiring relatively high power (in excess of 100 watts), the potential list of power applications is not limited and this invention can be applied to a wide range of power requirements.
FIG. 1 shows a conceptual representation of a prior art resonant inductive power transmission system. A source of alternating electrical energy is applied to the primary of a loosely coupled, air gap transformer. Magnetic coupling between the transformer primary and the transformer secondary transfers some proportion of the primary side energy to the transformer secondary, which is removed by some distance from the primary. The magnitude of the magnetic field generated by the primary is proportional to the current flowing in the primary winding. For this reason, it is highly desirable to use resonance to increase the magnitude of the primary winding currents and in this way to maximize the magnitude of that portion of the primary winding magnetic field that is linked into or coupled into the secondary.
The magnetic flux from the primary induces a voltage into the secondary winding. Maximum secondary current and therefore maximum power transmission occurs when the secondary winding is resonant as well. The result is a two-pole resonant circuit consisting of two magnetically coupled resonant circuits. The resonant circuits can be parallel resonant with the inductor and capacitor wired in parallel or they can be series wired and series resonant. Furthermore the primary and secondary side resonances need not share the same form.
Resonant inductive power transfer provides a means for the wireless transference of electrical power. The most common application for such technology is for the wireless recharging of batteries. In its most common configuration, alternating current with a line frequency of 50-60 Hz is drawn from the electrical grid, converted to direct current and converted again to alternating current but at a frequency much higher than line frequency. Inductive transmission frequencies in the range of 20-100 kHz are commonly used. The conversion from line frequency to the much higher inductive transmission frequencies is necessary in order to reduce the size and weight of the wireless transmission inductive components.
FIG. 1 is a conceptual block diagram of a prior art resonant inductive wireless power transfer system. Alternating line current is rectified by line frequency rectifier 10 and ripple filtered by line frequency ripple filter 12 to convert the alternating lines current into direct current that is applied to a DC-to-AC inverter 14 that generates high frequency alternating current at the transmission transformer operating frequency. Transmission transformer 16 is an air core transformer having primary and secondary windings. In this diagrammatic representation, it also includes primary and secondary side resonating capacitors. On the secondary side of the transmission transformer 16, induced current is rectified by high frequency rectifier 18 and ripple filtered by high frequency ripple filter 20 thereby converting it into direct current that is applied to the load 22, usually a battery.
FIG. 1 also shows the system waveforms present at the interfaces between functional blocks. Waveform conversion proceeds as follows: Line Frequency AC→Rectified Line Frequency AC→DC→High Frequency AC→Rectified High Frequency AC→DC.
The final result of the waveform conversion chain shown in FIG. 1 is direct current, used in many wireless power applications for battery charging. However, in some wireless power transmission applications the desired end product is line frequency AC which, according to conventional art, may be implemented by incorporating an additional DC-AC inverter 24 waveform conversion stage, converting direct current into alternating current of the desired frequency as shown in prior art FIG. 2 for application to a line frequency AC load 26. There are many methods of direct current to line frequency alternating current conversion known to skilled practitioners of the arts. The most basic approach converts the dc current into a line frequency square wave which is then filtered into a sinusoid, or more commonly applied un-filtered to the AC load 26 in lieu of a sine wave with the sometimes harmful effects of the square wave harmonic content.
Multiple alternate DC-to-AC conversion methods have been developed that approximate the desired sinusoidal AC voltages to various degrees of accuracy. These include rectangular waveforms with positive, negative and zero voltage intervals, staircase waveforms with multiple output voltage levels and pulse width modulation waveforms that given sufficient time and amplitude resolution can generate arbitrarily good approximations of a sinusoidal output waveform. However, without special provisions, the frequency the AC waveform provided by these DC-to-AC conversion schemes is derived locally and is not synchronized with the line frequency. Another limitation arises because DC-to-AC inverters that generate low distortion sinusoidal output are unavoidably complex on the circuit level. The invention described herein avoids these limitations.